In the fabrication of semiconductor devices, contact holes and other features that are formed on a substrate are filled with a conductive material to provide contacts and other circuitry. Electroless plating techniques are used to deposit copper, silver, platinum, and other conductive materials to fill openings.
A typical plating process to fill a contact opening involves depositing a seed layer (e.g., copper) onto a barrier layer formed over the substrate within the opening, and plating a layer of metal (e.g., copper) onto the substrate by applying an electrical potential between the seed layer and another electrode in the presence of an electroless solution containing an inorganic metal salt and a reducing agent.
Electroless deposition requires active surfaces that are compatible to nucleation and growth by using a metastable system that will heterogeneously nucleate on these selective areas by allowing the reducing agent and the complexed metal to find a favorable site to react. A problem encountered with electroless deposition methods is that surface potential of the feature heavily impacts the fill or plating rate.
In filling multiple electrically isolated features on a substrate by electroless deposition, the differences in surface potential between features can be problematic. The surface potential of features can differ, for example, based on the structure of the features (e.g., hole, elevated structure, etc.), whether the feature is strapped to an active area or to an inactive area in a substrate (or electrically isolated from the active area), the exposure of the feature to light which can generate a bias across a substrate, among other factors. Such differences in surface potential between features can result in uneven activation and significantly different deposition rates from one feature to the next. In traditional plating, low surface potential areas such as holes do not plate as quickly and may receive little or no plating at all. Often, when holes are present in the article to be plated, an uneven fill is deposited in the hole, which is thicker at the top and bottom of the hole than at the center of the hole. This can lead to defects in the circuits.
Therefore, it would be desirable to provide a process that overcomes such problems.